I am less surprised. Not all Debian architectures have ocamlopt. The
architectures where ocamlopt is available are documented in
/usr/lib/ocaml/native-archs (you should use OCAML_HAVE_OCAMLOPT or
OCAML_OPT_ARCH from /usr/share/ocaml/ocamlvars.mk, if needed, in your
debian/rules).
It's the maintainer's job to make packages work even if ocamlopt is missing.

Thank you for your reply. Will do. Please don't be in too much a hurry...
All the best,
Guillaume Yziquel.